1. Field of the Invention
The present invention relates to a plastic package type semiconductor device, more particularly, to a plastic package type semiconductor device in which a semiconductor integrated circuit chip is encapsulated in a molded layer of an epoxy resin composition.
2. Description of the Related Art
In semiconductor devices in recent years, a marked improvement is being achieved in the density of the elements and the operating .speed of the devices, as seen from ASIC (Application Specific IC), with the result that the power consumption tends to be increased in these semiconductor devices. With increase in the power consumption, an excellent heat dissipation performance is required for the package of the semiconductor device.
It was customary in the past to use an epoxy resin as an encapsulating resin in a plastic package type semiconductor device. However, the conventional epoxy resin exhibits such a low heat conductivity as about 0.1 to 0.3 W/m.multidot.K, resulting in failure to achieve a sufficient heat dissipation. To overcome the difficulty, inorganic fillers are added to the epoxy resin so as to improve the heat conductivity of the resin. For example, the conventional encapsulating resin composition is provided by a thermosetting resin composition prepared by adding to an epoxy resin a fused silica powder as an inorganic filler, a curing agent, etc.
However, the heat conductivity of the fused silica is about 0.7 W/m.multidot.K, which is not sufficiently high, with the result that the epoxy resin composition containing the fused silica fails to exhibit a sufficiently high heat conductivity.
To impart a sufficient heat conductivity to an epoxy resin, it is proposed to add to the epoxy resin inorganic fillers having a heat conductivity higher than that of fused silica. For example, it is proposed to add inorganic fillers such as beryllium oxide, aluminum nitride, magnesium oxide, boron nitride and silicon nitride to the epoxy resin. However, particles of these inorganic fillers having a high heat conductivity have in general a hardness greater than that of the fused silica particle. The high hardness of these inorganic fillers causes the molding die to be severely abraded in the molding step of the encapsulating resin.
What should also be noted is that a resin composition containing inorganic fillers is generally low in its flowability. It follows that the molding efficiency is low in any of the case of using an epoxy resin composition containing the fused silica and the case of using an epoxy resin composition containing the inorganic filler having a high heat conductivity.
As described above, the conventional plastic package type semiconductor device leaves much room for further improvement. Specifically, it is very difficult to prevent the moldability from being impaired and the molding die from being abraded in the molding step in the manufacture of a semiconductor device encapsulated with an epoxy resin composition containing an inorganic filler which is intended to enable the resin composition to exhibit a sufficient heat dissipation.